Nature of Non-Volcanic Microtremor Observed on the Aso Caldera

Seismic measurements at over seventy points on the Aso caldera have revealed the nature of stationary ground tremor on the atrio area, which cannot be attributed to traffic passage nor to the volcanic activity.The tremor is sinusoidal and stationary, the wave characteristics suffering little temporal change at any fixed point. On the contrary, it changes remarkably with change of observation location.The spatial trend of observed waveform has a definite correlation with the topography, which suggests the possibility of its usefulness to structural interpretations of the Aso caldera.The particle motion at the ground surface is observed to be confined in a horizontal plane. The spatial phase relation of the tremor shows that it is not a simple standing wave phenomenon but has a complicated propagating nature and that the particle motion is transversal to the direction of wave propagation.